This invention relates generally to oilfield perforating and fracturing using explosive shaped charges and is particularly concerned with a method of forming non-circular perforations in hydrocarbon-bearing subterranean formations using a uniquely designed shaped charge perforator having multiple initiation points.
After a well has been drilled and casing has been cemented in the well, perforations are created in the casing, cement liner and surrounding formation to provide paths or tunnels in the formation through which oil and gas can flow toward the well, through the holes in the cement liner and casing and into the wellbore for transportation to the surface. These perforations are typically cylindrical or round holes made by conventional explosive shaped charge perforators. Usually, these perforators are tightly arranged in helical patterns around downhole tools called well perforators or perforating guns, which are lowered into the wellbore adjacent the target oil and gas producing formations. Once in place the shaped charges are detonated, thereby making multiple holes in the well casing, cement liner and surrounding target formation. In many cases hundreds of these charges are detonated sequentially in rapid succession to produce a large number of perforations that penetrate radially in all directions into the target formation.
Conventional shaped charge perforators typically include a cup-shaped metal case or housing having an open end, a high explosive charge disposed inside the case, and a thin concave metallic liner closing the open end. The case has a base portion that is configured to receive a detonator cord, which also is connected to the base portion of the other shaped charges so that a large number of charges can be detonated nearly simultaneously. Each shaped charge is typically detonated by initiating the explosive charge with the detonating cord at a single location at the back of the base portion of the case, usually at a point on the central horizontal axis of the case. The resultant detonation wave collapses the metal liner to form a forward moving high velocity jet that travels out of the open end of the case. The jet is a highly focused metal penetrator in which all the energy is focused in a single line. The jet, traveling at speeds on the order of about 7 km/s, pierces the well casing and the cement liner and forms a cylindrical tunnel in the surrounding target formation. Conventional shaped charge perforators usually produce circular tunnels having a diameter typically less than about one inch.
After holes have been formed by the shaped charge perforators in the formation, a highly viscous fracturing fluid containing a propping agent is often pumped into the formation to hydraulically fracture the rock and prop the fractures open, thereby creating a permeable flow path through which oil and gas can enter the wellbore. A typical problem often encountered when fracturing through the circular tunnels made by conventional shaped charge perforators is that the circular holes have a tendency to bridge with the propping agents causing what is known as “screen-outs” to occur in the fracturing process. These “screen outs” frequently cause the fracturing treatment to be halted. It is known that circular hole diameters must be at least six times the median proppant diameter to avoid bridging and the resultant “screen outs” that create operational problems. It is also known that, if the holes created in the formation are in the shape of a slot, the width of the slot must only be 2.5 to 3 times the median proppant diameter to avoid bridging by the propping agent. The smaller perforation requirement of the slot results in penetrations that may expose greater formation surface, thereby increasing production. Also, for a given slot width, a larger proppant can be used to create more permeable fractures that allow for easier oil and gas flow.
It has been proposed to create slotted perforations in oil and gas formations by using linear shaped charges to create the perforations. However, the use of prior art linear shaped charges has several disadvantages. First, because of geometry, the linear jets produced by such charges produce poor formation penetration. Second, the tools used for producing linear jets are very different from conventional designs and therefore require additional training of personnel and increase the probability of expensive mistakes. Finally, the perforator guns for carrying the linear charges are very complex and create the potential for mechanical failure that could result in expensive repairs or even loss of the well.
It is clear from the above discussion that a method for creating linear or slotted perforations using explosive shaped charge perforators of a more conventional design as compared to that of a linear shaped charge is desirable.